GENERAL

History will recognise the vital role played by D.C. motors in the development of industrial power transmission systems. The D.C. machine was the first practical device to convert electrical power into mechanical power, and vice versa in its generator form. Inherently straightforward operating characteristics, flexible performance and high efficiency encouraged the widespread use of D.C. motors in many types of industrial drive application.

The later developments of the lower-cost A.C. cage motor and, more recently, of electronic variable-frequency control have displaced the D.C. motor to some extent, particularly in the lower kW range. Nevertheless, the advantages associated with the inherently stable and relatively simple to control D.C. machine are indisputable. In its most straightforward form, speed is approximately proportional to armature voltage, torque to armature current and there is a one-to-one relationship between starting torque and starting current.

Modern D.C. motors under thyristor control and with sophisticated protection continue to provide very sound industrial variable-speed drive performance. High-performance test rigs and the higher-kW ratings of drives for the printing and paper industries, for high-speed passenger lifts, and drives subject to high transient loading in the metal and plastics industries; all are likely to continue employing the D.C. motor with thyristor control for some considerable time particularly in refurbishment programmes where a D.C. motor exists. The task facing the A.C. drive to completely oust its D.C. competitor is formidable.

D.C. machine tool and servo drives based mainly on chopper technology continue to offer high performance at...